Warping is a significant problem for flip chip ball grid array (BGA) packages. Generally, warping reduces the planarity of the package and makes external and internal connections of the package more difficult and less dependable. In particular, the stress that causes the warping typically varies with the temperature so that thermal cycling of a package can make connections unreliable in the area of the warping.
A common operation in the manufacture of a flip chip BGA package is the attachment of a stiffener or heat spreader around the perimeter of the substrate in the package. The same structure typically acts as both a stiffener and a heat spreader to improve the mechanical and thermal performance of the package. To simplify terminology, the term stiffener is used herein to designate such a structure whether the primary purpose of the structure is to improve mechanical or thermal performance of a package.
A typical process for attaching a stiffener applies a thermally activated adhesive to the substrate and/or the stiffener, places the stiffener and substrate in contact with the adhesive sandwiched between the stiffener and substrate, and applies pressure and heat. Control or monitoring of the process ensures that the resulting product is flat and uniform in appearance and/or ensures that the separation between the substrate and stiffener is uniform.
Adhesive generally must be kept from escaping or protruding from the edge of the package to avoid changing the package outline from the required design standard. A key control of the attachment process keeps the adhesive well back from the edge of the package to prevent the escape of adhesive. However, edge effects of the adhesive can cause stress that warps the substrate and can affect the outer solder balls in the BGA. Accordingly, methods for manufacturing a flip chip BGA package and attaching a stiffener without creating stress and warping that affects the BGA are desired.
In accordance with an aspect of the invention, an attachment process controls the minimum extent of the adhesive that attaches a stiffener to a substrate in a BGA flip-chip package. In particular, the edge of the adhesive is controlled to be beyond the centers of the outermost balls in the ball grid array. Accordingly, edge effects that may warp the substrate at the edge of the adhesive do not affect the ball grid array.
One specific embodiment of the invention is a process for fabricating an integrated circuit package. The process includes: creating a composite structure including the substrate, an adhesive layer, and a stiffener; and controlling an edge of the adhesive layer to ensure that a distance from an edge of the integrated circuit package to the edge of the adhesive is less than a distance from the edge of the integrated circuit package to a center of an outermost solder ball on the substrate. The stiffener can change the tensile characteristics of the package and/or act as a heat spreader to improve the thermal characteristics of the package. Conventional techniques for controlling the amount and placement of the adhesive between the stiffener and the substrate and the pressure applied to the composite structure to force the adhesive toward the edge of the package can place the edge of the adhesive outside the outermost solder balls in the ball grid array.